Branching filter and communication device

ABSTRACT

A branching filter and a communication device have excellent characteristics and an optimized configuration of a transmitting filter and a receiving filter. The branching filter includes a transmitting filter and a receiving filter wherein piezoelectric thin film resonators including a piezoelectric thin film sandwiched between opposed electrodes are arranged in a ladder type configuration on an opening or a recess of a substrate. The transmitting filter and the receiving filter are connected in parallel to an antenna terminal. The piezoelectric thin film resonators defining the transmitting filter and the piezoelectric thin film resonators defining the receiving filter are different from each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a branching filter including filtershaving piezoelectric thin film resonators, and the branching filter ispreferably for use in a communication device, for example.

2. Description of the Related Art

In recent years, piezoelectric thin film filters using elastic bulkwaves have been developed.

Such piezoelectric thin film filters are compact in size, are lightweight, and have excellent vibration resistance and impact resistance.In addition, the piezoelectric thin film filters have small variation inproducts and high reliability, and can provide non-adjusting circuits.Therefore, the mounting process can be automated and simplified.Furthermore, even when the frequency is increased, the piezoelectricthin film filters can be easily produced. Thus, the piezoelectric thinfilm filters have superior characteristics.

A branching filter (duplexer) including such piezoelectric thin filmfilters has been proposed.

For example, Japanese Unexamined Patent Application Publication No.2001-24476 discloses a branching filter including piezoelectric thinfilm filters in which the piezoelectric thin film resonators arearranged in a ladder configuration.

The piezoelectric thin film resonators used in the branching filterdisclosed in Japanese Unexamined Patent Application Publication No.2001-24476 define a transmitting filter and a receiving filter. In bothfilters, the electrodes are composed of Mo and piezoelectric thin filmsare composed of AlN.

However, the required characteristics are different between thetransmitting filter and the receiving filter in the branching filter.

In other words, piezoelectric thin film resonators having the samestructure are optimized only in either the transmitting filter or thereceiving filter.

According to the Japanese Unexamined Patent Application Publication No.2001-24476, the transmitting filter and the receiving filter have thesame structure. Therefore, a branching filter having optimumcharacteristics in both transmitting and receiving cannot be achieved.

SUMMARY OF THE INVENTION

In view of the problems described above, preferred embodiments of thepresent invention provide a branching filter having excellentcharacteristics in which the configuration of the transmitting filterand the receiving filter is optimized.

In order to solve the above problems, a branching filter of the presentinvention includes a transmitting filter and a receiving filter whereinpiezoelectric thin film resonators including at least one piezoelectricthin film sandwiched between at least one pair of opposed electrodes arearranged in a ladder configuration on an opening or a recess of asubstrate, the transmitting filter and the receiving filter beingconnected to an antenna terminal in parallel. In the branching filter,the piezoelectric thin film resonators defining the transmitting filterand the piezoelectric thin film resonators defining the receiving filterhave a different structure.

In the branching filter of preferred embodiments of the presentinvention, the piezoelectric thin film resonators defining thetransmitting filter and the piezoelectric thin film resonators definingthe receiving filter preferably include a different piezoelectric film.

In the branching filter of preferred embodiments of the presentinvention, the piezoelectric film of the piezoelectric thin filmresonators defining the transmitting filter is preferably composed ofAlN and the piezoelectric film of the piezoelectric thin film resonatorsdefining the receiving filter is preferably composed of ZnO.

In the branching filter of preferred embodiments of the presentinvention, the material of the electrodes is preferably differentbetween the piezoelectric thin film resonators defining the transmittingfilter and the piezoelectric thin film resonators defining the receivingfilter.

In the branching filter of preferred embodiments of the presentinvention, the acoustic impedance of the material of the electrodes ispreferably different between the piezoelectric thin film resonatorsdefining the transmitting filter and the piezoelectric thin filmresonators defining the receiving filter.

In the branching filter of preferred embodiments of the presentinvention, the frequency of the pass band of the receiving filter ispreferably higher than the frequency of the pass band of thetransmitting filter, and the acoustic impedance of the material of theelectrodes defining the receiving filter is preferably higher than theacoustic impedance of the material of the electrodes defining thetransmitting filter.

In the branching filter of preferred embodiments of the presentinvention, the piezoelectric thin film resonators defining thetransmitting filter preferably use second harmonic waves and thepiezoelectric thin film resonators defining the receiving filterpreferably use fundamental waves.

In the branching filter of preferred embodiments of the presentinvention, in addition to the above-described configuration, thepiezoelectric thin film resonators defining the transmitting filter andthe piezoelectric thin film resonators defining the receiving filterpreferably include a different insulating film on the opening or therecess of the substrate.

In the branching filter of preferred embodiments of the presentinvention, the insulating film of the piezoelectric thin film resonatorsdefining the receiving filter is preferably composed of SiO₂.

In the branching filter of preferred embodiments of the presentinvention, the insulating film of the piezoelectric thin film resonatorsdefining the receiving filter is preferably composed of two layersincluding an SiO₂ layer adjacent to the piezoelectric thin film and anAl₂O₃ layer adjacent to the SiO₂ layer.

In the branching filter of preferred embodiments of the presentinvention, the insulating film of the piezoelectric thin film resonatorsdefining the receiving filter is preferably composed of two layersincluding an SiO₂ layer adjacent to the piezoelectric thin film and anAlN layer adjacent to the SiO₂ layer.

In the branching filter of preferred embodiments of the presentinvention, the insulating film of the piezoelectric thin film resonatorsdefining the transmitting filter is preferably composed of two layersincluding an AlN layer adjacent to the piezoelectric thin film and anSiO₂ layer adjacent to the AlN layer.

In the branching filter of preferred embodiments of the presentinvention, the insulating film of the piezoelectric thin film resonatorsdefining the transmitting filter is preferably composed of two layersincluding an Al₂O₃ layer adjacent to the piezoelectric thin film and anSiO₂ layer adjacent to the Al₂O₃ layer.

A communication device of another preferred embodiment of the presentinvention includes the branching filter according to one of thepreferred embodiments described above.

The branching filter of various preferred embodiments of the presentinvention preferably includes a transmitting filter and a receivingfilter wherein piezoelectric thin film resonators including at least onepiezoelectric thin film sandwiched between at least one pair of opposedelectrodes are arranged in a ladder configuration on an opening or arecess of a substrate, the transmitting filter and the receiving filterbeing connected to an antenna terminal in parallel. In the branchingfilter, the piezoelectric thin film resonators defining the transmittingfilter and the piezoelectric thin film resonators defining the receivingfilter have a different structure.

According to the above-described unique configuration, the transmittingfilter and the receiving filter include piezoelectric thin filmresonators having different structures from each other. As a result, abranching filter having optimum characteristics in both of thetransmitting filter and the receiving filter can be advantageouslyprovided.

Other features, elements, characteristics and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a duplexer according to a preferredembodiment of the present invention.

FIG. 2 is a schematic cross-sectional view showing the structure of aresonator of a transmitting filter in the duplexer.

FIG. 3 is a schematic cross-sectional view showing the structure of aresonator of a receiving filter in the duplexer.

FIG. 4 is a schematic cross-sectional view showing the structure of aresonator of a receiving filter according to a preferred embodiment ofthe present invention.

FIG. 5 is a graph showing the displacement of vibration of each layer inan example of the resonator in FIG. 4.

FIG. 6 is a graph showing the relationship between the electromechanicalcoupling coefficient k² _(eff) and the film thickness ratio in theresonator in FIG. 4.

FIG. 7 is a graph showing the relationship between Q factor and the filmthickness ratio in the resonator in FIG. 4.

FIG. 8 is a graph showing the relationship between the temperaturecoefficient of frequency (TCF) and the film thickness ratio in theresonator in FIG. 4.

FIG. 9 is a schematic cross-sectional view showing the structure of theresonator in a transmitting filter according to a preferred embodimentof the present invention.

FIG. 10 is a graph showing the displacement of vibration of each layerin an example of the resonator in FIG. 9.

FIG. 11 is a graph showing the relationship between k² _(eff) and thefilm thickness ratio in the resonator in FIG. 9.

FIG. 12 is a graph showing the relationship between Q factor and thefilm thickness ratio in the resonator in FIG. 9.

FIG. 13 is a graph showing the relationship between TCF and the filmthickness ratio in the resonator in FIG. 9.

FIG. 14 is a circuit diagram showing a modification of the duplexer.

FIG. 15 is a circuit diagram showing a modification of the duplexer.

FIG. 16 is a circuit diagram showing a modification of the duplexer.

FIG. 17 is a schematic cross-sectional view showing a modification ofthe resonator in the transmitting filter and the receiving filter.

FIG. 18 is a graph showing the frequency characteristics of insertionloss in a transmitting filter and a receiving filter according to apreferred embodiment of the present invention.

FIG. 19 is a graph showing the frequency characteristics of insertionloss in a transmitting filter and a receiving filter according to apreferred embodiment of the present invention.

FIG. 20 is a graph showing the frequency characteristics of insertionloss in a transmitting filter and a receiving filter in a comparativeexample.

FIG. 21 is a graph showing the frequency characteristics of insertionloss in a transmitting filter and a receiving filter in a comparativeexample.

FIG. 22 is a cross-sectional view of a piezoelectric thin film resonatorused in a fourth preferred embodiment of the present invention.

FIG. 23 is a graph showing an investigation result of theelectromechanical coupling coefficient with the piezoelectric filmthickness ratio in the fourth preferred embodiment of the presentinvention.

FIG. 24 is a circuit block diagram of a communication device including aduplexer according to various preferred embodiments of the presentinvention.

FIG. 25 is a schematic cross-sectional view showing the structure of aresonator of a receiving filter according to a preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Preferred Embodiment

A preferred embodiment of the present invention will now be describedwith reference to FIGS. 1 to 3.

In the present preferred embodiment, a duplexer in which thetransmission band is about 1,850 MHz to about 1,910 MHz and thereception band is about 1,930 MHz to about 1,990 MHz will now bedescribed.

As shown in FIG. 1, the duplexer (branching filter) according to thepresent preferred embodiment preferably includes a transmitting terminal1, a receiving terminal 2, and an antenna terminal 3.

The duplexer preferably includes a transmitting filter 5, a receivingfilter 6, and a matching circuit 7. The transmitting filter 5 isdisposed between the antenna terminal 3 and the transmitting terminal 1.The receiving filter 6 is disposed between the antenna terminal 3 andthe receiving terminal 2. The matching circuit 7 is disposed between theantenna terminal 3 and the receiving filter 6.

In other words, in the duplexer, the transmitting filter 5 and thereceiving filter 6 are connected to the antenna terminal 3 in parallel.

A capacitance 8 is disposed between the antenna terminal 3 and thetransmitting filter 5.

The pass band in the transmitting filter 5 and that in the receivingfilter 6 are set so as to be different from each other.

The transmitting filter 5 includes series resonators 11 a and 11 b andparallel resonators 12 a and 12 b arranged in a ladder configuration.

The parallel resonators 12 a and 12 b are grounded through inductances13 a and 13 b.

The inductances 13 a and 13 b can extend the pass band of thetransmitting filter 5.

The receiving filter 6 includes series resonators 21 a, 21 b, and 21 cand parallel resonators 22 a, 22 b, 22 c, and 22 d arranged in a ladderconfiguration.

The parallel resonators 22 a, 22 b, 22 c, and 22 d are grounded.

The matching circuit 7 includes an inductance 71 connected in series andcapacitances 72 and 73 connected in parallel.

In the present preferred embodiment, the resonators in the transmittingfilter 5 and the receiving filter 6 are piezoelectric thin filmresonators. Each of the piezoelectric thin film resonators includes athin film (piezoelectric thin film) composed of a piezoelectric materialand electrodes that sandwich the piezoelectric thin film and are opposedto each other.

The characteristics desired for the transmitting filter 5 and thereceiving filter 6 wherein the transmitting filter has relatively lowfrequency characteristics and the receiving filter has relatively highfrequency characteristics will now be described.

A large electric power is applied to the transmitting filter 5.

Therefore, the resonators used in the transmitting filter 5 preferablyhave a high Q factor.

This Q factor represents the mechanical vibration loss in a resonator.

Since a low Q factor increases the mechanical vibration loss in theresonator, the loss causes heat and the resonator generates heat. As aresult, the lifetime of the resonator is shortened.

Furthermore, the lifetime of the transmitting filter 5 is alsoshortened.

The Q factor depends on the structure of the resonator. Furthermore, thesmaller the elastic loss of the material used in the resonator, thehigher the Q factor is.

Since the elastic loss of materials also depends on the frequency etc.,it is difficult to mention the specific values. However, propagationloss, which is often used in, for example, a surface acoustic wavedevice, is an indicator.

That is, the smaller the propagation loss of the material used in theresonator, the higher the Q factor of the resonator is.

A material having a high thermal conductivity is preferably used as theresonators in the transmitting filter 5.

The reason for this is as follows: A low thermal conductivity decreasesthe heat dissipation effect. As a result, the resonator is heated andthe lifetime of the resonator is shortened.

The electromechanical coupling coefficient k² (effective couplingcoefficient k² _(eff)) of the resonators in the transmitting filter 5 ispreferably about 3% to about 4%.

The reason for this is as follows: Even when the electromechanicalcoupling coefficient k² _(eff) is small, the pass band can be extendedto the low frequency side to some degree with an external circuit (forexample, an extended inductance).

When the electromechanical coupling coefficient k² _(eff) is about 5% ormore, roll-off characteristics at the high frequency side (i.e., thesteepness of the attenuation in the range from the pass band of about1,910 MHz in the transmitting to the pass band of about 1,930 MHz in thereceiving) is deteriorated.

The use of a material having a large electromechanical couplingcoefficient k² as the piezoelectric thin film increases the k² _(eff) ofthe resonator.

The electromechanical coupling coefficient k² _(eff) also depends on thestructure of the resonator.

In the receiving filter 6, when the pass band is extended to the lowfrequency side with an external circuit, the receiving filter 6interferes with the transmitting filter 5.

In addition, an external circuit cannot extend the pass band to the highfrequency side.

For these reasons, in the receiving filter 6, a predetermined filterband must be provided using resonators having a large electromechanicalcoupling coefficient k² _(eff) and without an auxiliary externalcircuit.

The structure of the piezoelectric thin film resonator of thetransmitting filter 5 and the piezoelectric thin film resonator of thereceiving filter 6 having the above-described characteristics will nowbe described in detail with reference to FIGS. 2 and 3.

As shown in FIG. 2, a resonator of the transmitting filter 5 includes asupporting substrate 32 preferably composed of silicon (Si) and aninsulating film 31 disposed on the supporting substrate 32.

Furthermore, the supporting substrate 32 includes an opening or hollowportion that penetrates the supporting substrate 32 in the direction ofthe thickness and extends to the other side of the insulating film 31.

A lower electrode 33, a piezoelectric thin film 34, and an upperelectrode 35 are disposed on the insulating film 31 in that order.

The insulating film 31 forms a diaphragm.

This diaphragm faces the opening or hollow portion.

As shown in FIG. 3, a resonator of the receiving filter 6 includes asupporting substrate 42 composed of silicon (Si) and an insulating film41 disposed on the supporting substrate 42.

Furthermore, the supporting substrate 42 includes an opening or hollowportion that penetrates the supporting substrate 42 in the direction ofthe thickness and extends to the insulating film 41.

A lower electrode 43, a piezoelectric thin film 44, and an upperelectrode 45 are disposed on the insulating film 41 in that order.

The insulating film 41 forms a diaphragm.

This diaphragm faces the opening or hollow portion.

Second harmonic waves are used in the resonators shown in FIGS. 2 and 3.

In the present preferred embodiment, the type of the piezoelectric thinfilm is different between the resonators of the transmitting filter 5and the resonators of the receiving filter 6.

In the resonators of the transmitting filter 5, the piezoelectric thinfilm 34 is preferably composed of AlN, the insulating film 31 ispreferably composed of SiO₂, and the lower electrode 33 and the upperelectrode 35 are preferably composed of Au/Ti.

In the resonators of the receiving filter 6, the piezoelectric thin film44 is preferably composed of ZnO, the insulating film 41 is preferablycomposed of SiO₂, and the lower electrode 43 and the upper electrode 45are preferably composed of Au/Ti.

The resonators of the transmitting filter 5 will now be described inmore detail.

Aluminum nitride (AlN) has a thermal conductivity that is higher thanthat of ZnO and has an elastic loss that is smaller than that of ZnO.

Aluminum nitride (AlN) has a small electromechanical couplingcoefficient (k_(t)=0.23, thermal conductivity W/(m·° C.)=150).

Accordingly, the resonators of the transmitting filter 5 have a Q factorand a heat dissipation effect higher than those of the resonators of thereceiving filter 6.

Furthermore, in the resonators of the transmitting filter 5, theinsulating film 31 composed of SiO₂ is preferably used. Therefore, thesign of the temperature coefficient of the insulating film 31 composedof SiO₂ and that of the piezoelectric thin film 34 composed of AlN areopposite with respect to each other.

Therefore, the temperature change is cancelled out in the piezoelectricthin film 34 and the insulating film 31. As a result, the temperaturecharacteristics in the resonators of the transmitting filter 5 can beimproved.

The acoustic velocity in AlN is larger than that in ZnO. In order toobtain a frequency that is equivalent to that of the resonator usingZnO, the film thickness of the diaphragm must be increased or anelectrode material having a large density must be used.

When the film thickness of AlN is increased, the area (vibrationportion) where the upper electrode 35 is overlapped with the lowerelectrode 33 must be increased in order that the capacitance (C₀) of theresonator is controlled to be a predetermined value. As a result, theelement size is increased.

However, a metal having a density of at least about 8 g/cm³ (forexample, Au: 19.3, Pt: 21.45, Ni: 8.9, and Mo: 10.4) can be used as atleast one of the upper electrode 35 and the lower electrode 33. In thiscase, the predetermined frequency can be obtained without increasing thearea of the upper electrode 35 or the lower electrode 33.

The resonators of the receiving filter 6 will now be described in moredetail.

Zinc oxide (ZnO) has an electromechanical coupling coefficient that islarger than that of AlN (k_(t)=0.30).

Accordingly, the resonators of the receiving filter 6 have a largeelectromechanical coupling coefficient k² _(eff).

Zinc oxide (ZnO) has a thermal conductivity lower than that of AlN(thermal conductivity W/(m·° C.)=4).

Furthermore, in the resonators of the receiving filter 6, the insulatingfilm 41 that is preferably composed of SiO₂ is used.

Therefore, the sign of the temperature coefficient of the insulatingfilm 41 composed of SiO₂ and that of the piezoelectric thin film 44composed of ZnO are opposite with respect to each other.

Therefore, the temperature change is cancelled out in the piezoelectricthin film 44 and the insulating film 41. As a result, the temperaturecharacteristics in the resonators of the receiving filter 6 can beimproved.

In the resonators of the transmitting filter 5, AlN having a smallelectromechanical coupling coefficient k² _(eff) is used as thepiezoelectric thin film 34. Therefore, the electromechanical couplingcoefficient k² _(eff) of the resonators of the transmitting filter issmaller than that of the resonators of the receiving filter.

As shown in FIG. 1, the inductances 13 a and 13 b are connected to theparallel resonators 12 a and 12 b of the transmitting filter 5.Therefore, in this case, the pass band can be extended to the lowfrequency side, thereby obtaining the desired bandwidth.

Second Preferred Embodiment

Another preferred embodiment of the present invention will now bedescribed with reference to FIGS. 4 to 8.

For the convenience of description, components having the same functionas those of the components shown in the first preferred embodiment havethe same reference numerals and the description is omitted.

In the present preferred embodiment, as shown in FIG. 4, an insulatingfilm 41 in the resonators of the receiving filter 6 is preferablycomposed of two layers: An insulating film 41 a is disposed on asubstrate 42 and an insulating film 41 b is disposed on the insulatingfilm 41 a.

In the present preferred embodiment, the insulating film 41 a ispreferably composed of Al₂O₃ and the insulating film 41 b is preferablycomposed of SiO₂.

In this structure, a compressive stress is applied on the piezoelectricthin film 44 composed of ZnO and the insulating film 41 b composed ofSiO₂, whereas a tensile stress is applied on the insulating film 41 acomposed of Al₂O₃.

This structure stabilizes the strength of the diaphragm.

In the present preferred embodiment, the insulating film 41 a may becomposed of AlN.

In this case, the sign of the temperature coefficient of the insulatingfilm 41 a composed of AlN and that of the insulating film 41 b composedof SiO₂ are opposite with respect to each other.

Therefore, the temperature change is cancelled out in the insulatingfilm 41 a and the insulating film 41 b. As a result, the temperaturecharacteristics in the resonators of the receiving filter 6 are greatlyimproved.

Furthermore, since AlN is superior in the thermal conductivity comparedwith Al₂O₃, the heat dissipation effect can be improved.

The above-described structure can increase the electromechanicalcoupling coefficient k² _(eff).

This is because the acoustic impedance of SiO₂ defining the insulatingfilm 41 b is about 1.3×10⁷ (N·s/m³), which is smaller than that of ZnO(about 3.5×10⁷ (N·s/m³)) defining the piezoelectric thin film 44, thatof Al₂O₃ (about 3.9×10⁷ (N·s/m³)) and that of AlN (about 3.5×10⁷(N·s/m³)) defining the insulating film 41 a.

In other words, acoustic waves are significantly reflected at theinterface between the piezoelectric thin film 44 and the insulating film41 b, and the energy of the acoustic waves is concentrated on thepiezoelectric thin film 44. Accordingly, the electromechanical couplingcoefficient k² _(eff) can be increased.

As shown in the displacement diagram of vibration in FIG. 5, thedisplacement of vibration in ZnO of the piezoelectric thin film 44 islarger than that in SiO₂ of the insulating film 41 b.

The thickness of the piezoelectric thin film 44, the insulating film 41a composed of Al₂O₃, and the insulating film 41 b composed of SiO₂ willnow be described. As shown in FIG. 6, in terms of largeelectromechanical coupling coefficient k² _(eff), the film thicknessratio represented by the thickness of the piezoelectric thin film 44:(the thickness of the insulating film 41 a composed of Al₂O₃+ thethickness of the insulating film 41 b composed of SiO₂) is preferablyabout 0.7 to about 1.3.

Furthermore, as shown in FIG. 7, in terms of high Q factor, the filmthickness ratio is preferably about 0.6 to about 0.8.

As shown in FIG. 8, in terms of small absolute value of the temperaturecoefficient of frequency (TCF), the film thickness ratio represented bythe insulating film 41 a (Al₂O₃): the insulating film 41 b (SiO₂) ispreferably about 1 or more.

However, when the ratio of the insulating film 41 a to the insulatingfilm 41 b is excessively small, the problem of stress balance occurs.Therefore, the film thickness ratio represented by the insulating film41 a (Al₂O₃): the insulating film 41 b (SiO₂) is more preferably about 1to about 3.

In FIGS. 6 to 8, the piezoelectric thin film 44 is preferably composedof ZnO, the insulating film 41 a is preferably composed of Al₂O₃, andthe insulating film 41 b is preferably composed of SiO₂.

The upper electrode 45 and the lower electrode 43 that sandwich thepiezoelectric thin film 44 are preferably composed of Al and have a filmthickness of about 180 nm.

The figures show the calculation results in which the film thicknessratio of the insulating film 41 b (SiO₂) to the insulating film 41 a(Al₂O₃) is varied from about 3:1 to about 1:3 under the above-describedconditions.

The absolute amount of each film thickness is determined such that thefrequency band of the resonators is controlled to be about 1,900 MHz.

Third Preferred Embodiment

A further preferred embodiment of the present invention will now bedescribed with reference to FIGS. 9 to 17.

For the convenience of description, components having the same functionas those of the components shown in the first preferred embodiment andthe second preferred embodiment have the same reference numerals and thedescription is omitted.

In the present preferred embodiment, as shown in FIG. 9, an insulatingfilm 31 in the resonators of the transmitting filter 5 is preferablycomposed of two layers: An insulating film 31 a is disposed on asubstrate 32 and an insulating film 31 b is disposed on the insulatingfilm 31 a.

In the present preferred embodiment, the insulating film 31 a ispreferably composed of SiO₂ and the insulating film 31 b is preferablycomposed of AlN.

In this case, since AlN is superior in the thermal conductivity, theheat dissipation effect of the element can be improved.

This structure can achieve high withstand power, extend the lifetime,and improve the reliability of the element.

In the present preferred embodiment, the insulating film 31 a may becomposed of SiO₂ and the insulating film 31 b may be composed of Al₂O₃.

In this structure, a compressive stress is applied on the insulatingfilm 31 a composed of SiO₂, whereas a tensile stress is applied on theinsulating film 31 b composed of Al₂O₃.

This structure can stabilize the strength of the diaphragm.

The above-described structure can decrease the absolute value of thetemperature coefficient of frequency (TCF).

The reason for this is as follows: The temperature coefficient of ZnO,Al₂O₃, and AlN that are used as the piezoelectric thin film 34 or theinsulating film 31 b is negative (i.e., the rise in temperaturedecreases the frequency). On the other hand, the temperature coefficientof SiO₂ used as the insulating film 31 a is positive.

As shown in the displacement diagram of vibration in FIG. 10 (whereinZnO is used as the piezoelectric thin film 34), the displacement ofvibration in ZnO of the piezoelectric thin film 34 is strongly affectedby the temperature coefficient of SiO₂ defining the insulating film 31a. As a result, the TCF of the whole resonator is shifted in thepositive direction (i.e., comes close to zero).

When the above-described piezoelectric thin film 34, the insulating film31 a composed of SiO₂, and the insulating film 31 b composed of Al₂O₃are used, the thickness of the insulating film 31 a and the insulatingfilm 31 b is preferably as follows. As shown in FIGS. 11 and 12, interms of large electromechanical coupling coefficient k² _(eff) and highQ factor, since the dependency to the thickness of the piezoelectricthin film 34 is small, the film thickness ratio is not particularlylimited. However, the film thickness ratio represented by the thicknessof the piezoelectric thin film 34: (the thickness of the insulating film31 a composed of SiO₂+ the thickness of the insulating film 31 bcomposed of Al₂O₃) is preferably about 0.7 to about 1.2.

As shown in FIG. 13, in terms of small absolute value of the temperaturecoefficient of frequency (TCF), the film thickness ratio represented bythe insulating film 31 a (SiO₂): the insulating film 31 b (Al₂O₃) ispreferably about 1 or more.

However, when the ratio of the insulating film 31 a (SiO₂) to theinsulating film 31 b (Al₂O₃) is excessively small, the problem of stressbalance occurs. Therefore, the film thickness ratio represented by theinsulating film 31 a (SiO₂): the insulating film 31 b (Al₂O₃) is morepreferably about 1 to about 3.

In FIGS. 11 to 13, the piezoelectric thin film 34 is preferably composedof ZnO, the insulating film 31 a is preferably composed of SiO₂, and theinsulating film 31 b is preferably composed of Al₂O₃.

The upper electrode 35 and the lower electrode 33 that sandwich thepiezoelectric thin film 34 are preferably composed of Al and preferablyhave a film thickness of about 180 nm.

The figures show the calculation results in which the film thicknessratio of the insulating film 31 b (Al₂O₃) to the insulating film 31 a(SiO₂) is varied from about 3:1 to about 1:3 under the above conditions.

The absolute amount of each film thickness is determined such that thefrequency band of the resonators is controlled to be about 1,900 MHz.

As shown in FIG. 14, the receiving filter may include two seriesresonators and three parallel resonators.

As shown in FIG. 15, in the transmitting filter, a resonator may beadded in series adjacent to the transmitting terminal. The matchingcircuit may include two inductances connected in series and acapacitance connected in parallel. Furthermore, the capacitance 8 may beomitted.

As shown in FIG. 16, each of the series resonators in the transmittingfilter in FIG. 14 may be replaced with two series resonators.

A modification of the resonator in the transmitting filter 5 and thereceiving filter 6 will now be described with reference to FIG. 17.

As shown in FIG. 17, the resonator includes an insulating film 51 on arecess 56 disposed on a substrate 52. The insulating film 51 issuspended over the recess 56 at the periphery.

A lower electrode 53, a piezoelectric thin film 54, and an upperelectrode 55 are disposed on the insulating film 51.

The above-described structures of the piezoelectric thin film and theinsulating film in the transmitting filter 5 and the receiving filter 6can be applied to this structure. Thus, the same advantages can beachieved in this structure.

In addition, when the resonators of the transmitting filter 5 and theresonators of receiving filter 6 are composed of the same materials andare different only in the deposited order, the same deposition equipmentcan be used to reduce the cost.

The transmitting filter 5 including resonators including a piezoelectricthin film 34 preferably composed of ZnO, an insulating film 31 apreferably composed of SiO₂, and an insulating film 31 b preferablycomposed of AlN can achieve a Q factor of about 700 and anelectromechanical coupling coefficient k² _(eff) of about 2.9%.

The receiving filter 6 including resonators having a piezoelectric thinfilm 44 composed of ZnO, an insulating film 41 a composed of Al₂O₃, andan insulating film 41 b composed of SiO₂ can achieve a Q factor of 400and an electromechanical coupling coefficient k² _(eff) of about 5.3%.

FIGS. 18 and 19 show the frequency characteristics of insertion loss inthe transmitting filter 5 and the receiving filter 6.

In the transmitting filter 5, the inductances are connected to theparallel resonators. Therefore, as shown in FIGS. 18 and 19, thebandwidth can be extended to the low frequency side despite the smallelectromechanical coupling coefficient k² _(eff).

In contrast, in the receiving filter 6, the bandwidth can be increasedbecause of the large electromechanical coupling coefficient k² _(eff).

As shown in FIG. 19, regarding the bandwidth wherein the level isattenuated by about 3.5 dB, the transmitting filter 5 can provide thebandwidth of about 80 MHz, and the receiving filter 6 can provide thebandwidth of about 68 MHz.

As a comparative example, a receiving filter 6 including resonatorshaving a piezoelectric thin film 44 preferably composed of ZnO, aninsulating film 41 a preferably composed of SiO₂, and an insulating film41 b preferably composed of AlN is used. The resonators have a Q factorof about 700 and an electromechanical coupling coefficient k² _(eff) ofabout 2.9%. As shown in FIGS. 20 and 21, in this receiving filter 6, thebandwidth wherein the level is attenuated by about 3.5 dB is no morethan about 36 MHz.

Fourth Preferred Embodiment

In the present preferred embodiment, the resonance characteristics in apiezoelectric thin film resonator 100 shown in FIG. 22 wereinvestigated.

The piezoelectric thin film resonator 100 includes a supportingsubstrate 102 preferably composed of silicon (Si).

A lower electrode 103, a piezoelectric thin film 104 composed of ZnO,and an upper electrode 105 are disposed on the supporting substrate 102in that order.

Furthermore, the supporting substrate 102 includes an opening or hollowportion that penetrates the supporting substrate 102 in the direction ofthe thickness and extends to the other side of the lower electrode 103.

A diaphragm facing the opening or hollow portion is formed.

In this experiment, the upper electrode 105 and the lower electrode 103in the piezoelectric thin film resonator 100 were composed of the samematerial and had the same film thickness.

The material of the electrodes used in this example was aluminum Al,molybdenum Mo, copper Cu, tungsten W, and platinum Pt.

FIG. 23 shows the investigation result of the relationship between apiezoelectric film thickness ratio and the electromechanical couplingcoefficient (k² _(eff)) concerning the various materials of theelectrodes. The piezoelectric film thickness ratio is the ratio of thefilm thickness of the piezoelectric thin film to the total filmthickness (the thickness of the upper electrode 105 + the thickness ofthe piezoelectric thin film 104 + the thickness of the lower electrode103) of the resonator.

As shown in FIG. 23, when the piezoelectric film thickness ratio wasoptimally selected, among the above-described five kinds of materials ofthe electrodes, the highest electromechanical coupling coefficient (k²_(eff)) was achieved with W, and subsequently, Pt, Mo, Cu, and Al, inthat order.

Table 1 shows the approximate acoustic impedance and the resistivity inthe materials of the electrodes.

TABLE 1 Material of Acoustic impedance Resistivity electrodes (Ns/m³)(μΩ cm) W 1.0 × 10⁸ 5.5 Pt 7.5 × 10⁷ 10.6 Mo 6.9 × 10⁷ 5.7 Cu 3.9 × 10⁷1.7 Al 1.7 × 10⁷ 2.7

Referring to FIG. 23 and Table 1, the higher the acoustic impedance ofthe material, the higher the electromechanical coupling coefficient (k²_(eff)) can be.

As shown in the first preferred embodiment to the third preferredembodiment, resonators having a large electromechanical couplingcoefficient (k² _(eff)) must be used in a filter (for example, receivingfilter) disposed at the high frequency side of the duplexer.

In a filter (for example, transmitting filter) disposed at the lowfrequency side, the band can be extended by providing an externalinductance. Therefore, even when resonators having a smallelectromechanical coupling coefficient (k² _(eff)) are used, thesteepness of the filter can be provided.

Accordingly, in the receiving filter, a material that has high acousticimpedance to increase the electromechanical coupling coefficient (k²_(eff)) is preferably used as the electrodes.

On the other hand, in the transmitting filter, copper or aluminum thathas low acoustic impedance but has a low resistivity is preferably usedto form the electrodes. Thus, a duplexer having excellentcharacteristics can be produced.

Fifth Preferred Embodiment

Another preferred embodiment of the present invention will now bedescribed with reference to FIG. 25.

For the convenience of description, components having the same functionas those of the components shown in the first preferred embodiment tothe fourth preferred embodiment have the same reference numerals and thedescription is omitted.

In the present preferred embodiment, as shown in FIG. 25, resonators ofa receiving filter 6 can use fundamental waves.

The resonators using fundamental waves can have an electromechanicalcoupling coefficient k² _(eff) that is larger than that of theresonators using second harmonic waves, which are shown in FIGS. 2 to 4and FIG. 9.

Consequently, a pass band required in the receiving filter can beprovided.

For example, a duplexer may include resonators of the receiving filter 6having a piezoelectric thin film preferably composed of AlN and usingfundamental waves, and resonators of the transmitting filter 5 having apiezoelectric thin film preferably composed of ZnO and using secondharmonic waves. Thus, a duplexer having excellent characteristics can beachieved.

A communication device using the duplexer described in theabove-described preferred embodiment will now be described withreference to FIG. 24.

At the receiving side (Rx side) wherein receiving is performed, thecommunication device 600 includes an antenna 601, an antenna common/RFTop filter 602, an amplifier 603, an Rx interstage filter 604, a mixer605, a 1st IF filter 606, a mixer 607, a 2nd IF filter 608, a 1st+2ndlocal synthesizer 611, a temperature compensated crystal oscillator(TCXO) 612, a divider 613, and a local filter 614.

As shown by the double lines in FIG. 24, the transmitting from the Rxinterstage filter 604 to the mixer 605 is preferably performed withbalanced signals so as to secure the balance.

At the transceiving side (Tx side) wherein transmitting is performed,the communication device 600 shares the antenna 601 and the antennacommon/RF Top filter 602, and includes a Tx IF filter 621, a mixer 622,a Tx interstage filter 623, an amplifier 624, a coupler 625, an isolator626, and an automatic power control (APC) 627.

The duplexer of the above-described preferred embodiment can be suitablyused as the Rx interstage filter 604 and the RF Top filter 602.

The present invention is not limited to the above-described preferredembodiments and various modifications are possible within the scopeshown in the claims. The technical field of the present invention alsoincludes embodiments obtained by appropriately combining technicalmethods disclosed in the different embodiments.

The branching filter including filters having piezoelectric thin filmresonators of the present invention can be applied to variouscommunication devices such as a cellular phone.

While the present invention has been described with respect to preferredembodiments, it will be apparent to those skilled in the art that thedisclosed invention may be modified in numerous ways and may assume manyembodiments other than those specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention which fall within the true spirit andscope of the invention.

1. A branching filter comprising: a transmitting filter; and a receivingfilter; wherein piezoelectric thin film resonators defining thetransmitting filter and the receiving filter and including at least onepiezoelectric thin film sandwiched between at least one pair of opposedelectrodes are arranged in a ladder configuration on an opening or arecess of a substrate, the transmitting filter and the receiving filterbeing connected to an antenna terminal in parallel; the piezoelectricthin film resonators defining the transmitting filter and thepiezoelectric thin film resonators defining the receiving filter aremade of different materials from each other; and the piezoelectric thinfilm resonators defining the transmitting filter and the piezoelectricthin film resonators defining the receiving filter have differentpiezoelectric films.
 2. The branching filter according to claim 1,wherein the piezoelectric film of the piezoelectric thin film resonatorsdefining the transmitting filter includes AlN and the piezoelectric filmof the piezoelectric thin film resonators defining the receiving filterincludes ZnO.
 3. The branching filter according to claim 1, wherein thematerial of the electrodes is different between the piezoelectric thinfilm resonators defining the transmitting filter and the piezoelectricthin film resonators defining the receiving filter.
 4. The branchingfilter according to claim 3, wherein the acoustic impedance of thematerial of the electrodes is different between the piezoelectric thinfilm resonators defining the transmitting filter and the piezoelectricthin film resonators defining the receiving filter.
 5. The branchingfilter according to claim 3, wherein the frequency of the pass band ofthe receiving filter is higher than the frequency of the pass band ofthe transmitting filter, and the acoustic impedance of the material ofthe electrodes defining the receiving filter is higher than the acousticimpedance of the material of the electrodes defining the transmittingfilter.
 6. A communication device comprising the branching filteraccording to claim
 1. 7. A branching filter comprising: a transmittingfilter; and a receiving filter; wherein piezoelectric thin filmresonators defining the transmitting filter and the receiving filter andincluding at least one piezoelectric thin film sandwiched between atleast one pair of opposed electrodes are arranged in a ladderconfiguration on an opening or a recess of a substrate, the transmittingfilter and the receiving filter being connected to an antenna terminalin parallel; the piezoelectric thin film resonators defining thetransmitting filter and the piezoelectric thin film resonators definingthe receiving filter are made of different materials from each other;and the piezoelectric thin film resonators defining the transmittingfilter use second harmonic waves and the piezoelectric thin filmresonators defining the receiving filter use fundamental waves.
 8. Abranching filter comprising: a transmitting filter; and a receivingfilter; wherein piezoelectric thin film resonators defining thetransmitting filter and the receiving filter and including at least onepiezoelectric thin film sandwiched between at least one pair of opposedelectrodes are arranged in a ladder configuration on an opening or arecess of a substrate, the transmitting filter and the receiving filterbeing connected to an antenna terminal in parallel; the piezoelectricthin film resonators defining the transmitting filter and thepiezoelectric thin film resonators defining the receiving filter aremade of different materials from each other; and the piezoelectric thinfilm resonators defining the transmitting filter and the piezoelectricthin film resonators defining the receiving filter further comprise adifferent insulating film on the opening or the recess of the substrate.9. The branching filter according to claim 8, wherein the insulatingfilm of the piezoelectric thin film resonators defining the receivingfilter comprises SiO₂.
 10. The branching filter according to claim 8,wherein the insulating film of the piezoelectric thin film resonatorsdefining the receiving filter comprises two layers including an SiO₂layer adjacent to the piezoelectric thin film and an Al₂O₃ layeradjacent to the SiO₂ layer.
 11. The branching filter according to claim8, wherein the insulating film of the piezoelectric thin film resonatorsdefining the receiving filter comprises two layers including an SiO₂layer adjacent to the piezoelectric thin film and an AlN layer adjacentto the SiO₂ layer.
 12. The branching filter according to claim 8,wherein the insulating film of the piezoelectric thin film resonatorsdefining the transmitting filter comprises two layers including an AlNlayer adjacent to the piezoelectric thin film and an SiO₂ layer adjacentto the AlN layer.
 13. The branching filter according to claim 8, whereinthe insulating film of the piezoelectric thin film resonators definingthe transmitting filter comprises two layers including an Al₂O₃ layeradjacent to the piezoelectric thin film and an SiO₂ layer adjacent tothe Al₂O₃ layer.
 14. A branching filter comprising: a transmittingfilter; and a receiving filter; wherein piezoelectric thin filmresonators defining the transmitting filter and the receiving filter andincluding at least one piezoelectric thin film sandwiched between atleast one pair of opposed electrodes are arranged in a ladderconfiguration on an opening or a recess of a substrate, the transmittingfilter and the receiving filter being connected to an antenna terminalin parallel; the piezoelectric thin film resonators defining thetransmitting filter and the piezoelectric thin film resonators definingthe receiving filter use different waves from each other; and thepiezoelectric thin film resonators defining the transmitting filter andthe piezoelectric thin film resonators defining the receiving filterhave different piezoelectric films.
 15. The branching filter accordingto claim 14, wherein the piezoelectric film of the piezoelectric thinfilm resonators defining the transmitting filter includes AlN and thepiezoelectric film of the piezoelectric thin film resonators definingthe receiving filter includes ZnO.
 16. The branching filter according toclaim 14, wherein the material of the electrodes is different betweenthe piezoelectric thin film resonators defining the transmitting filterand the piezoelectric thin film resonators defining the receivingfilter.
 17. The branching filter according to claim 16, wherein theacoustic impedance of the material of the electrodes is differentbetween the piezoelectric thin film resonators defining the transmittingfilter and the piezoelectric thin film resonators defining the receivingfilter.
 18. The branching filter according to claim 16, wherein thefrequency of the pass band of the receiving filter is higher than thefrequency of the pass band of the transmitting filter, and the acousticimpedance of the material of the electrodes defining the receivingfilter is higher than the acoustic impedance of the material of theelectrodes defining the transmitting filter.
 19. A communication devicecomprising the branching filter according to claim
 14. 20. A branchingfilter comprising: a transmitting filter; and a receiving filter;wherein piezoelectric thin film resonators defining the transmittingfilter and the receiving filter and including at least one piezoelectricthin film sandwiched between at least one pair of opposed electrodes arearranged in a ladder configuration on an opening or a recess of asubstrate, the transmitting filter and the receiving filter beingconnected to an antenna terminal in parallel; the piezoelectric thinfilm resonators defining the transmitting filter and the piezoelectricthin film resonators defining the receiving filter use different wavesfrom each other; and the piezoelectric thin film resonators defining thetransmitting filter use second harmonic waves and the piezoelectric thinfilm resonators defining the receiving filter use fundamental waves. 21.A branching filter comprising: a transmitting filter; and a receivingfilter; wherein piezoelectric thin film resonators defining thetransmitting filter and the receiving filter and including at least onepiezoelectric thin film sandwiched between at least one pair of opposedelectrodes are arranged in a ladder configuration on an opening or arecess of a substrate, the transmitting filter and the receiving filterbeing connected to an antenna terminal in parallel; the piezoelectricthin film resonators defining the transmitting filter and thepiezoelectric thin film resonators defining the receiving filter usedifferent waves from each other; and the piezoelectric thin filmresonators defining the transmitting filter and the piezoelectric thinfilm resonators defining the receiving filter further comprise adifferent insulating film on the opening or the recess of the substrate.22. The branching filter according to claim 21, wherein the insulatingfilm of the piezoelectric thin film resonators defining the receivingfilter comprises SiO₂.
 23. The branching filter according to claim 21,wherein the insulating film of the piezoelectric thin film resonatorsdefining the receiving filter comprises two layers including an SiO₂layer adjacent to the piezoelectric thin film and an Al₂O₃ layeradjacent to the SiO₂ layer.
 24. The branching filter according to claim21, wherein the insulating film of the piezoelectric thin filmresonators defining the receiving filter comprises two layers includingan SiO₂ layer adjacent to the piezoelectric thin film and an AlN layeradjacent to the SiO₂ layer.
 25. The branching filter according to claim21, wherein the insulating film of the piezoelectric thin filmresonators defining the transmitting filter comprises two layersincluding an AlN layer adjacent to the piezoelectric thin film and anSiO₂ layer adjacent to the AlN layer.
 26. The branching filter accordingto claim 21, wherein the insulating film of the piezoelectric thin filmresonators defining the transmitting filter comprises two layersincluding an Al₂O₃ layer adjacent to the piezoelectric thin film and anSiO₂ layer adjacent to the Al₂O₃ layer.